Not all genetic variations are associated with threats or harms to human health. Some even  protect us, such as genetic variations that have been shown to make bones harder or the heart more impervious to disease. But while some genetic variations are positive, others can cause or contribute to disease. In this episode, we answer the question of how does gene therapy work, and learn how gene therapy replaces and repairs certain gene variants, and is changing the trajectory of genetic diseases.For several years, Dr. Jean Bennett at the University of Pennsylvania’s Department of  Ophthalmology Center for Advanced Retinal and Ocular Therapeutics investigated the possibility of replacing gene variants in the retina – which cause blindness – with copies of healthy ones.After successfully treating blind puppies, Dr. Bennett and her colleagues turned their attention to treating human eyesight.The retina turned out to be a good target site for gene therapy. In most parts of the human  body, our cells keep dividing after we’re born. But not the rods and cones in our retina – these photoreceptor cells don’t regenerate.A major obstacle in the development of gene therapy is the tendency of cloned genes to get lost in the process of cell division before they have a chance to integrate into the host DNA. Because retinal cells don't divide after birth, the cloned gene may be expressed for a prolonged timeIn 2017, after decades of painstaking research, building on the efforts of countless scientists  throughout history, Dr. Bennett and her colleagues had the evidence that gene therapy may be used to treat genetic conditions in humans.Since then, gene therapy has undergone giant leaps in the treatment of specific diseases. And record numbers of gene therapy trials are ongoing, including potential treatments for conditions such as sickle cell disease and Parkinson’s disease. It’s just one of many ways that the field of gene therapy is poised to change the world in the years to come.